Squamous epithelium refers to a specific type of body cell found on various external and internal body and organ surfaces. Squamous epithelium has covering or protective properties and is therefore also known as covering epithelium.
What is squamous epithelium?
Epithelial tissue is composed of cells lined up individually, but the shape and thickness of the rows formed vary depending on the body region and function. Therefore, different types of squamous epithelium are known. The epithelial cells, which usually lie flat, are strongly interconnected and therefore form a covering and protective layer. Epithelial tissue of all types is therefore considered particularly robust and stable. In the center of each epithelial cell is typically a cell nucleus, nucleus. In the cytoplasm of each squamous cell are the so-called cell organelles, which are responsible for the metabolic performance of each cell. The nucleus contains the genome with the genetic information in the form of a DNA strand as a double helix. Typical cell organelles in each squamous cell are, for example, the endoplasmic reticulum, the Golgi apparatus, the ribosomes and the mitochondria as power plants of each cell. In the histology laboratory, differentiation of the different cell layers of squamous epithelium is easily possible. Histology, i.e., the fine-tissue examination of squamous epithelia, plays a particularly important role in pathology when it comes to diagnosing inflammatory changes or cell proliferation.
Anatomy and structure
In all types of squamous tissue, the uppermost cell layer is usually irregularly shaped and typically closely interlocked. This interlocking, recognized as mosaic-like, occurs through so-called tight junctions and other motile binding proteins that provide tremendous, virtually unbreakable binding within squamous cells. Basically, a distinction must be made anatomically between single-layered and multilayered as well as between keratinizing and non-keratinizing squamous epithelium. In some organ systems, the squamous epithelium has adapted to the special anatomical conditions in such a way that special function-specific anatomical designations have been formed from it. For example, the multi-rowed non-keratinizing squamous epithelium of the entire urogenital tract is called urothelium. The uncorneous squamous epithelium of the respiratory tract is also called cylindrical epithelium because of its typical shape. The entire outer skin of a human being consists of keratinizing, multi-layered squamous epithelium and is considered to be particularly stable in its protective effect against the outside world due to the additional incorporation of collagen fibers. The horny layer is formed by the continuous death of so-called keratinocytes, horny cells. This keratinization is another property of certain squamous epithelia that can be used anatomically for differentiation.
Function and tasks
Squamous epithelium, in its different variations and manifestations, has important protective and covering functions on the surface of organs, organ systems, and vessels. Squamous epithelium, however, does not perform the function of so-called parenchyma, the actual organ functional cells. Single-layered, unkeratinized squamous epithelium forms, for example, the boundary of the alveoli, the alveoli of the lungs. Without squamous epithelium on the surface of the alveoli, gas exchange would not be possible due to the lack of surface tension. Several layers of the single-layered squamous epithelium are also found in the membranous labyrinth of the inner ear. There, the epithelium is significantly involved in the transmission of sound waves as well as in the maintenance of the sense of balance. The entire mucosa of the oral cavity consists of multilayered, unkeratinized squamous epithelium. Due to the permanent wetting with saliva, the main function there is also a protective function as a rough barrier against germs or blunt impacts during food intake. The entire esophagus is also equipped with the multi-layered squamous epithelium on the inside. In this way, the food pulp can be transported muscularly active and yet safely into the stomach. Multi-layered keratinized squamous epithelium forms the uppermost skin layer of the outer skin, also called epidermis. Due to its multilayered structure, the epidermis is the most important entry barrier against external influences. Due to the close-meshed structure of the epidermis, bacteria, viruses or fungi cannot penetrate an intact skin surface.
Diseases
Epithelium exhibits a particularly high rate of mitosis and proliferation. However, it is precisely this circumstance that makes squamous epithelium comparatively susceptible to disorders and diseases. Only an intact squamous epithelium, whether in the form of mucosa or skin, can fully perform its protective, supporting, and covering functions. Even slight mucosal defects can become entry points for pathogens, resulting in serious infections. This does not only refer to defects in the squamous epithelium of the epidermis, but also to defects of the squamous epithelium in the body. The most common clinical pictures that are directly related to changes in the squamous epithelium include inflammations as well as benign and malignant tumors. Inflammation of the squamous epithelium is characterized by the 5 so-called cardinal symptoms of rubor, calor, dolor, tumor, and functio laesa. Thus, in addition to redness and swelling, physiological function is always disturbed. In the case of pneumonia, this leads to a restriction of gas exchange, or in the case of inflammation of the urothelium, to discomfort during urination. Malignant tumors that originate directly from the squamous epithelium are common and are called squamous cell carcinomas. They are among the most common human tumor neoplasms and often exhibit invasive growth and a tendency to metastasize. Typical squamous cell carcinomas include esophageal carcinoma, pleural mesothelioma, and anal marginal carcinoma. Early detection is crucial for the prognosis of all squamous cell carcinomas. As long as squamous cell carcinoma does not grow invasively and has not formed daughter tumors, it is considered curable. However, metastatic squamous cell carcinoma is responsible for a large proportion of cancer deaths in Western industrialized nations.
